This arena bristled in mutinous mood last night, yet Arsenal's progress in the gathering maelstrom was serene. Victory has hoisted the London side back to within four points of Chelsea at the top with a chance to vault above Manchester United into second place tantalisingly close against a managerless Bolton Wanderers next midweek. The campaign feels as promising for Arsenal as it is proving treacherous for Portsmouth.

As a contest, the Premier League's first not to feature an Englishman in either starting line-up, this degenerated into a mismatch from the moment the hosts were first breached. Pompey offered huff and puff where Arsenal were incisive and inventive, the home side slipping into disarray in the face of such quality. Alex Song's late header, converted emphatically but too easily amid static defenders, summed up the gulf in class.

Another win next Wednesday will leave Arsène Wenger's side a point behind the leaders. "The belief is there," the Frenchman said. "This season is completely open, from Chelsea to Manchester City or even Liverpool [in seventh], everybody could still be there because anyone can drop points." This win felt all the more impressive given Cesc Fábregas's absence with a hamstring injury – albeit now for only 10 days rather than the initial prognosis of three weeks – but, without the Spaniard, a Welshman revelled.

Aaron Ramsey had been outstanding here long before he rasped in his side's third midway through the second period, the 19-year-old's left-foot shot ripping beyond a despairing Asmir Begovic. "Aaron is a player who can play right-back, left wing, in central midfield," Wenger said of the former Cardiff player, a loser to Portsmouth in the 2008 FA Cup final. "He is an all-rounder. Cesc is a playmaker. But what they have in common when they are on the pitch is the personality to say: 'I'm not scared to play.'"

Fear is increasingly gripping Portsmouth's game. The traumas that have long gripped this club off the pitch are unavoidable on it these days. The locals, so dismayed at the latest wretched turn in their club's sparse fortune, were reduced to spitting abuse at those representatives of their latest owners, sitting shivering in the directors' box, long before the end, though even their calls of "Sack the board" and "You're not fit to run the club" felt tired. Arsenal's optimism seemed incongruous amid discord.

Something appeared to snap here. Portsmouth did dismantle Liverpool earlier this month but could not have expected to undermine Arsenal. Yet, following the unnerving news of Her Majesty's Revenue and Customs's winding-up petition, their patience with those in the boardroom has clearly expired. Every backward pass or wayward attempt mustered by their side fuelled the discontent in the stands. The captain, Aaron Mokoena, was booked for a foul on Ramsey with many of the home support pleading with the referee, Alan Wiley, to send him off. His subsequent substitution was cheered. This was less pantomime, more poison.

The South African had been guilty of a similar foul in prompting the visitors' opening goal, though the concession was farcical. Hermann Hreidarsson had back-pedalled towards his left-hand post as Eduardo da Silva prepared to take a free-kick some 25 yards out but, as the Croat meandered up to strike, the Icelandic defender inexplicably hopped away from the upright. Eduardo's attempt was still tame but flicked off Younes Kaboul and Hreidarsson, uncertain of his position, merely turned to see it nestle in the net.

Nasri added a second, finishing slickly after Eduardo and Ramsey had ferried the ball expertly across the area, and Pompey were cut adrift. Grant, an increasingly beleaguered figure on the sidelines, claimed not to have noticed either the abuse that rained down on Mokoena or upon his employers in the directors' box.

"It is not easy, but I know the board are trying their best," he said, though he continues to cling to the slim prospect of having funds to spend in next month's transfer window. "I hope to. No one has told me that Manchester City want to sign Kaboul. All the players in the team need to stay, and I think the board know that. We need the players to stay and we need to make the team stronger." Neither appears likely.

Nadir Belhadj's side-footed riposte offered scant consolation, with the Algerian one of four players to be absent until February at the African Cup of Nations and this squad already heaving under the strain and four points adrift. These clubs are apparently careering in very different directions.